This invention relates to electrically conductive elastomeric ink compositions, particularly for use in electrical and electronic circuitry and keyboard switches and assemblies. More particularly, the invention is directed to switch arrays wherein one or more of the switch contacts or conductive traces on a printed circuit board are defined by a novel conductive ink.
An increasing number of keyboards are being provided with switch control devices formed of elastomeric springs, typically as an array of silicone rubber domes. The underside of the dome has a protrusion or contactor portion provided with an outer conductive dot or layer. Upon depressing a key the conductive dot shorts a pair of contacts formed on a printed circuit board or other substrate which the individual key switch assembly overlies. Switch assemblies of this type are disclosed in commonly assigned copending U.S. patent application Ser. No. 479,540, filed Mar. 28, 1983.
Although the prior art recognizes the possibility of using electroconductive printing inks to form the conductive dots on switch contactors, adhesion of the ink to addition cured silicone polymers is difficult, particularly when the ink contains a high loading of conductive fillers. In commercial practice printing processes have not been used for this purpose to Applicant's knowledge, but rather conductive polymers, for instance epoxy loaded with silver flake or silicone loaded with carbon black, have been cured in sheets and then individual conductive dots punched from the sheet and manually bonded with an adhesive to the contactor protrusions on the underside of the domed silicone spring. Alternatively, molded conductive dot layers had been placed in a mold and comolded with the elastomeric spring. These processes are not only labor intensive but the conductive dot compositions suffer from relatively low electrical conductivity and oftentimes suffer from excessive contact bounce.
U.S. Pat. No. 4,264,477 (Seeger et al.) discloses an electrically conductive ink which may be screened and may be employed either as the contactor or contacts for a keyboard switch array. The ink composition comprises -325 mesh (or 44 micron) silver or other metal particle, e.g., nickel, or metal coated particles e.g., silver coated copper or silver coated glass, in admixture with a polyester, epoxy or vinyl polymeric binder and from 0.5 to about 12.5 percent of carbon black powder to provide reduced bounce during low current operation.
The prior art also discloses molded variable resistance devices comprising silicone rubber loaded with fine particulate materials such as carbon, tungsten and nickel-chromium. U.S. Pat. No. 3,918,020 (DuRocher) teaches such molded resistors formed from 7 parts by weight 140-200 mesh nichrome per 1 part of silicone rubber. Additional pressure-sensitive elastic resistor compositions are taught in U.S. Pat. Nos. 4,028,276 to Harden et al and 4,152,304 to Tadewald.
The present invention has as its primary object the provision of a flexible, elastomeric conductive ink composition of high conductivity. Another object is to provide such a composition which is compatible with, and adherent to or which can be made adherent to with an interlayer, a silicone elastomer substrate. Another object is to provide such a conductive composition which can be used as an inexpensive screening ink for printing circuit boards or the like. It is a further object to provide such composition which has a reduced cost by utilizing a minimum amount of conductive metal particles, which can be processed easily, and which has long life and minimal bounce even after millions of contact cycles.